1. Field of the Invention
The field of art to which this invention relates is cycloolefin copolymer (COC) resins for use in precision optical applications.
2. Description of the Prior Art
Resins comprising one or more cycloolefin copolymers have been disclosed for use in a variety of optical applications, including, for example, optical or compact disks, information recording films and base boards, optical fibers, and the like. U.S. Pat. No. 5,439,722, for example, discloses a cycloolefin copolymer comprising at least one cycloolefin copolymer for recording media, wherein the cycloolefin copolymer has a molar-mass distribution Mw/Mn<about 2, a molecular weight of less than/equal to about 30,000 g/mole and a glass transition temperature of from about 100° C. to 200° C. Also disclosed in U.S. Pat. No. 5,439,722 are blends of two or more COCs.
COC compositions for use in information recording films and information recording baseboards are disclosed in U.S. Pat. No. 4,874,808. More particularly, the patent discloses compositions comprising (A) a cycloolefin type random copolymer containing an ethylene component and a cycloolefin component represented by the general formula wherein n and m are each 0 or a positive integer, I is an integer of at least 3, R1 to R10 each represent a hydrogen atom, halogen atom or hydrocarbon group, wherein said cycloolefin type random copolymer (A) has an intrinsic viscosity of 0.05-10 dl/g as measured at 135° C. in decalin and a softening temperature of not lower than 70° C. and (B) a cycloolefin type random copolymer containing an ethylene component and a cycloolefin component represented by either of the aforesaid general formulas, wherein said cycloolefin type random copolymer (B) has an intrinsic viscosity of 0.01-5 dl/g as measured at 135° C. in decalin and a softening temperature of below 70° C., wherein the weight ratio of component (A) to component (B) ranges from 100/0.1 to 100/10. The combination of component (A) and component (B) is disclosed as providing compositions having improved adhesion. The patent also discloses that, in addition to the cycloolefin type random copolymer (A) and the cycloolefin type random copolymer (B), the composition may contain stabilizers such as phenolic antioxidants, metallic salts of fatty acids, and fatty esters of polyhydric alcohols. At column 15, lines 3 to 19, it is stated that “ . . . phenolic antioxidants as illustrated above are used in an amount of 0.01-10 parts by weight, preferably 0.05-3 parts by weight based on 100 parts by weight of the cycloolefin type random copolymer composition. Similarly, the fatty esters of polyhydric alcohol are used in an amount of 0.01-10 parts by weight, preferably 0.05-3 parts by weight based on 100 parts by weight of the said compositions.”
U.S. Pat. No. 4,874,808 specifically exemplifies compositions containing a cycloolefin copolymer (A) having a softening temperature ranging from 153° C. to 155° C., a cycloolefin copolymer (B) having a softening temperature ranging from 39° C. to 45° C.), 0.5% of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (a phenolic antioxidant), 0.05% of zinc stearate and 0.5% of glycerin monostearate, said amounts of phenolic antioxidant, zinc stearate, and glycerin monostearate being based on the total weight of the COC components (A) and (B). The patent reported adhesion and transparency data, but not silver streaking and color hue data for these compositions.
U.S. Pat. No. 4,874,808 further exemplifies compositions containing a single cycloolefin type random copolymer, 0.05% by weight of zinc stearate, 0.6% by weight of a phenolic antioxidant, and 0.6 to 9.6% by weight of a fatty ester of a polyhydric alcohol, which was either partially esterified, as in the case of Examples 16 to 18, or completely esterified, as in the case of Example 21. Example 19 of the patent exemplifies a composition containing a single cycloolefin type random copolymer, 0.6% by weight of a phenolic antioxidant and 0.05% by weight of zinc stearate; Example 22 exemplifies a composition containing a single cycloolefin type random copolymer, a partially esterified fatty ester of a polyhydric alcohol and zinc stearate. Molded specimens made from the compositions containing the single cycloolefin type random copolymer were evaluated for properties that included color hue (b value) and the formation of silver streaks. Specimens made from the compositions that contained the phenolic antioxidant, a partially esterified fatty ester of a polyhydric alcohol and zinc stearate (i.e., the compositions of Examples 16 to 18) exhibited less silver streaking and yellowing, as a function of the color hue value “b”, than specimens made from the compositions which lacked the antioxidant (i.e., the composition of Example 22) or specimens made from the compositions which lacked the fatty acid ester of the polyhydric alcohol (i.e., the composition of Example 19) or specimens that contained a completely esterified fatty acid ester of a polyhydric alcohol (i.e., the composition of Example 21). In particular, specimens made from the compositions of Examples 16 to 18 had color hue (b values) ranging from 1.0 to 1.2; in contrast, specimens made from the compositions of Examples 19, 21 and 22 had b values ranging from 3.0 to 5.0.
For optical applications requiring extreme clarity it is desirable to minimize silver streaking, and yellowing. On the Hunter color scale, yellowing is represented by the Hunter b color value. For precision optical applications, such as for example, optical display base boards, light enhancing plates, precision lenses light emitting diodes, light guides, and the like, cycloolefin copolymer resins having Hunter b color values of less than 1.0 are desired. Throughout the remainder of the specification and claims, “Hunter b color value” refers to the b value of a material tested in accordance with the “Hunter Color Test Procedure” set forth below.